abbrev mode .. on by default

[Harry Putnam]

Long ago someone posted a message from which I learned to turn on
abbrev mode by default by putting these two in .
emacs.

  (setq default-abbrev-mode t)
  (read-abbrev-file)

I understood if an ~/.abbrev_defs file was in place then the mode
would be on by default.

However, when I open a perl script I still have to turn it on. Is
there something more to set?

There is quite a lot in  C-h i m emacs <RET> about this but scanning
through with `index' seach I didn't notice a section detailing how to
turn this on by default.